Rashba Coefficient Research Articles

First‐Principle Studies of the Enhancement of Rashba Effect in Ultrathin GeTe Films by Structural Engineering

It is well known that electronic energy bands are split by spin-orbit coupling in a system that lacks inversion center, and the bulk Rashba effect in ferroelectric semiconductor GeTe has attracted intensive attention due to its giant Rashba coefficient. However, the Rashba effect in GeTe film will disappear below a critical thickness due to the occurrence of centrosymmetric structure with decreasing thickness. In this paper, GeTe/In 2 Se 3 $_2\text{Se}_3$ heterostructures are investigated by means of first-principle calculations, and it is found that the Rashba effect not only can be revived for the nonpolar-GeTe film, but also can be enhanced for the ultrathin polar-GeTe film. The variation of the bias field within the GeTe film in the heterostructure in contrast with that within the isolated GeTe film may be employed to explain the remarkable improvement of Rashba effect and the spin textures around the spin-split band. Furthermore, the influence of the In 2 Se 3 $_2\text{Se}_3$ polarization on the Rashba coefficient is also discussed. With the increasing demands of device miniaturization, the authors hope that the method of constructing heterostructure may be an alternative route to enable large Rashba effect surviving down to the ultrathin film limit.

Giant and tunable Rashba spin splitting in MoS2/Bi2Te3 heterostructures

The study of the spin-orbit coupling (SOC) effect is an active research field in physics and it is of great interest for materials with large Rashba spin splitting (RSS) in spintronics applications. In this report, based on first-principles calculations, we propose the heterostructure of MoS2/Bi2Te3 with giant RSS duo to the lack of spatial inversion symmetry and the intrinsic electric field perpendicular to the heterostructure. Furthermore, it is found that the electronic and RSS can be tuned effectively by layers of MoS2 and Bi2Te3. When the Bi2Te3 thickness is less than 2QL, these MoS2/Bi2Te3 heterojunction systems exhibit semiconductor properties. The magnitude of the RSS in these MoS2/Bi2Te3 heterojunctions changes with the thickness, and the largest band splitting is found in the (MoS2)1(Bi2Te3)2 heterostructure with a Rashba coefficient of ~2.5 eV Å, which is a very large value among two-dimensional (2D) materials. While the interlayer distance and charge transfer remain stable at about 3.5 Å and 0.1 e, respectively. Through layer-projected band structure analysis, it is proven that the electronic states of these quantum well states (QWSs) are mainly contributed by Bi2Te3, while the MoS2 layer only plays an inductive role and does not contribute to the QWSs electrons. In addition, strain only plays a minor role in the Rashba effect in the MoS2/Bi2Te3 heterojunction, which is mainly caused by the breaking of the spatial inversion symmetry. The giant and tunable Rashba-type SOC of the MoS2/Bi2Te3 heterostructure suggests that it is a type of promising materials for spintronic applications.

Perpendicular Magnetic Anisotropy and Its Electric Field Manipulation in Magnetic Multilayered Heterostructures

"Understanding of underlying physics related to the Perpendicular Magnetic Anisotropy (PMA) in magnetic heterostructures represents a major issue for its exploit in random-access memory (MRAM) devices. Using ab-initio analysis, we reveal some basic aspects related to the anatomy of PMA and its variation with electric field in various X/Fe/MgO(001) multilayer configurations (X=Cr, Au, V, Ag, Pt, Pd,…) compatible with standard experimental architectures of magnetic tunnel junction devices. Our study quantifies and underlines the significant role of the Rashba interfacial field on PMA. We explain and correlate the sign, the magnitude, and the electric field dependence of the PMA, the Rashba coefficient R and the Dzyaloshinskii–Moriya (DMI) asymmetric exchange interaction parameter. Moreover, when varying the Fe thickness in X/Fe/MgO(001) systems, we observe oscillations of PMA with the number of Fe monolayers, explained within the framework of quantum wells of the 1 Bloch symmetry electrons in Fe. Further atomistic micromagnetic simulations including different Fe layer thicknesses and the corresponding PMA predict macroscopic magnetization characteristics in realistic experimental systems. Keywords: perpendicular magnetic anisotropy, electric field control of PMA, magnetic tunnel junctions, magnetic multilayer heterostructures, atomistic magnetic simulations. "

Tuning the Rashba spin splitting in Janus MoSeTe and WSeTe van der Waals heterostructures by vertical strain

In this work, the electronic and spintronic properties of the five various stacking orders of bilayer MoSeTe, bilayer WSeTe, and MoSeTe/WSeTe van der Waals heterostructure under vertical strain is investigated using the first-principles calculations. It is observed that bilayer WSeTe is the most stable material and AA and AB stack patterns are the most stable configurations. The bandgap of the structures tends to decrease by increasing compressive strain and transition from semiconductor to metal occurs under specific compressive strain. In considered Janus TMD compounds, obvious out-of-plane asymmetry endows intrinsic electric potential difference and induces Rashba type spin-splitting at the Γ-point of valance band. Moreover, vertical strain can considerably improve the Rashba coefficient values in the different stacking patterns. Finally, our results could lead to a full understanding of the electronic and spintronic features of the WSeTe/MoSeTe, bilayer MoSeTe, and bilayer WSeTe compounds for application in spintronic devices.

Nonreciprocal Transport in a Rashba Ferromagnet, Delafossite PdCoO2.

Rashba interfaces yield efficient spin-charge interconversion and give rise to nonreciprocal transport phenomena. Here, we report magnetotransport experiments in few-nanometer-thick films of PdCoO2, a delafossite oxide known to display a large Rashba splitting and surface ferromagnetism. By analyzing the angle dependence of the first- and second-harmonic longitudinal and transverse resistivities, we identify a Rashba-driven unidirectional magnetoresistance that competes with the anomalous Nernst effect below the Curie point. We estimate a Rashba coefficient of 0.75 ± 0.3 eV Å and argue that our results qualify delafossites as a new family of oxides for nanospintronics and spin-orbitronics, beyond perovskite materials.

Origin of Rashba Spin Splitting and Strain Tunability in Ferroelectric Bulk CsPbF3

Spin-orbit coupling (SOC) in conjunction with broken inversion symmetry acts as a key ingredient for several intriguing quantum phenomena, viz., Rashba-Dresselhaus (RD) effect. The coexistence of spontaneous polarization and the RD effect in ferroelectric (FE) materials enables the electrical control of spin degrees of freedom. Here, we explore the FE lead halide perovskite CsPbF3 as a potential candidate in the field of spintronics by employing state-of-the-art first-principles-based methodologies, viz., density functional theory (DFT) with semilocal and hybrid functional (HSE06) combined with SOC and many-body perturbation theory (G0W0). For a deeper understanding of the observed spin splitting, the spin textures are analyzed using the k.p model Hamiltonian. We find there is no out-of-plane spin component indicating that the Rashba splitting dominates over Dresselhaus splitting. We also observe that the strength of Rashba spin splitting can be substantially tuned on application of uniaxial strain (±5%). More interestingly, we notice reversible spin textures by switching the FE polarization in CsPbF3 perovskite, making it potent for perovskite-based spintronic applications.

Rashba states situated inside the band gap of InTe/PtSe2 heterostructure

The Rashba effect of two-dimensional (2D) materials draws significant attention because this can boost the development of novel spintronic devices. For the purpose of practical applications, the ideal Rashba states should be formed inside the band gap. However, the reports on ideal Rashba states in 2D materials are still scarce. Here, first-principles calculations are utilized to verify the Rashba spin-orbit coupling of the 2D InTe/PtSe2 van der Waals heterostructure. The occurrence of internal electric field leads to Rashba spin splitting at the valence band edge near the Γ point. These Rashba states are entirely located inside the large band gap, which helps to employ them in practical usages like spintronic devices. We also explore the influence of in-plane strains, interlayer coupling, and external electric fields on the Rashba spin splitting. The Rashba coefficients are increased linearly by the tensile strains, while the compressed strains suppress the Rashba spin splitting. The strong interlayer coupling can also promote the Rashba spin splitting. Compared to the strains, the influence of the external electric field on the Rashba effect is not noteworthy. The synchronous change of Rashba coefficients and dipole moments suggests that the internal electric field is a key factor to affect the Rashba spin splitting.

Intertwined nontrivial band topology and giant Rashba spin splitting

Composite quantum compounds (CQCs) have become an important avenue for the investigation of intercorrelation between two distinct phenomena in physics. Topological superconductors, axion insulators, etc., are a few such CQCs which have recently drawn tremendous attention in the community. Topological nontriviality and Rashba spin physics are two different quantum phenomena but can be intertwined within a CQC platform. In this paper, we present a general symmetry-based mechanism, supported by ab initio calculations, to achieve intertwined giant Rashba splitting and topological nontrivial states simultaneously in a single crystalline system. Such coexistent properties can further be tuned to achieve other rich phenomena. We have achieved Rashba splitting energy ($\mathrm{\ensuremath{\Delta}}E$) and Rashba coefficient ($\ensuremath{\alpha}$) values as large as 161 meV and 4.87 eV \AA{}, respectively, in conjunction with the Weyl semimetal phase in ${\mathrm{KSnSb}}_{0.625}{\mathrm{Bi}}_{0.375}$. Interestingly, these values are even larger than the values reported for the widely studied topologically trivial Rashba semiconductor BiTeI. The advantage of our present analysis is that one can achieve various topological phases without compromising the Rashba parameters, within this CQC platform.

Interfacial Rashba band splitting in the organohalide lead perovskites: an ab-initio study

Halide perovskites have attracted significant attention in the past few years due to the spectacular growth in the efficiency of perovskite-based devices. The charge transfer at the metal oxide/perovskite interface is one of the most crucial device operational mechanisms. Understanding the interface features and their impact on device efficiency is becoming increasingly important. The built-in interfacial potential governs the charge transfer direction and rate. Concurrently, a spin–orbit coupling joint to the local electric field at the interface can cause a Rashba–Dresselhaus band splitting. Here, the geometrical and electronic structure of the interface between different terminated perovskite layers and titania is explored employing ab-initio techniques. The obtained results reveal higher stability and more beneficial band alignment for the PbI-terminated perovskite/titania interface. By estimating the electric field at the interface, the model slab perovskite response to the external field has been studied systematically. Rashba coefficients have been calculated for both conduction and valence bands. These results indicate a tunable band splitting at the presence of an external field or structural symmetry breaking. Our findings could help to understand the interfacial effects and further develop spin-based devices through Rashba-like effects.

Strain Effects on Rashba Spin-Orbit Coupling of 2D Hole Gases in GeSn/Ge Heterostructures.

A demonstration of 2D hole gases in GeSn/Ge heterostructures with a mobility as high as 20 000cm2 V-1 s-1 is given. Both the Shubnikov-de Haas oscillations and integer quantum Hall effect are observed, indicating high sample quality. The Rashba spin-orbit coupling (SOC) is investigated via magneto-transport. Further, a transition from weak localization to weak anti-localization is observed, which shows the tunability of the SOC strength by gating. The magneto-transport data are fitted to the Hikami-Larkin-Nagaoka formula. The phase-coherence and spin-relaxation times, as well as spin-splitting energy and Rashba coefficient of the k-cubic term, are extracted. The analysis reveals that the effects of strain and confinement potential at a high fraction of Sn suppress the Rashba SOC caused by the GeSn/Geheterostructures.

Persistent spin texture in tetragonal BiFeO3

We report the existence of bulk persistent spin texture in P4mm tetragonal BiFeO3, a ferroelectric with ultrahigh spontaneous polarization. Our density functional theory calculations reveal that tetragonal BiFeO3 exhibits sufficiently strong spin–orbit coupling, yielding a sizable Rashba coefficient of 0.31 eV · Å. More importantly, it is demonstrated that the polarization-induced spin–orbit field is unidirectional on the Z–Γ–X plane which contains the polar axis. Consequently, there emerges a momentum-independent spin texture, allowing a long-lived persistent spin helix with a wavelength of 23 nm in the real space.

Full spin-orbit coefficient in III-V semiconductor wires based on the anisotropy of weak localization under in-plane magnetic field

Because of the one-dimensional confinement of electron momentum in narrow semiconductor wires, spin relaxation is suppressed irrespective of the presence of spin-orbit (SO) interaction. In quantum transport, weak localization corrections to conductivity are reflected as suppressed spin relaxation, which makes quantification of the SO strength difficult because quantum correction theory requires weak antilocalization when evaluating SO coefficients. Narrow wires with strong SO interaction are potential platform for Majorana particles and parafermions for topological electronics and quantum computation, so revealing the SO strength in semiconductor wire structures is beneficial. Herein, we present quantification of the full SO coefficient under weak localization in InGaAs-based narrow wires. Using anisotropic weak localization observed under an in-plane external magnetic field with various orientations, one can ascertain the relative ratio between Rashba ($\ensuremath{\alpha}$) and linear Dresselhaus (${\ensuremath{\beta}}_{1}$) SO coefficients with no fitting. Furthermore, we find that widely tuning the potential profile of the quantum well through the top gate can expose a Rashba-predominant region in magnetoconductance, where the $\ensuremath{\alpha}$ value can be extracted reliably from two-dimensional quantum correction theory. Finally, we quantify the full SO coefficients including Rashba, linear Dresselhaus, and cubic Dresselhaus terms in the wire.

Two-dimensional Janus semiconductor BiTeCl and BiTeBr monolayers: a first-principles study on their tunable electronic properties via an electric field and mechanical strain.

Motivated by the recent successful synthesis of highly crystalline ultrathin BiTeCl and BiTeBr layered sheets [Debarati Hajra et al., ACS Nano, 2020, 14, 15626], herein for the first time, we carry out a comprehensive study on the structural and electronic properties of BiTeCl and BiTeBr Janus monolayers using density functional theory (DFT) calculations. Different structural and electronic parameters including the lattice constant, bond lengths, layer thickness in the z-direction, different interatomic angles, work function, charge density difference, cohesive energy and Rashba coefficients are determined to acquire a deep understanding of these monolayers. The calculations show good stability of the studied single layers. BiTeCl and BiTeBr monolayers are semiconductors with electronic bandgaps of 0.83 and 0.80 eV, respectively. The results also show that the semiconductor-metal transformation can be induced by increasing the number of layers. In addition, the engineering of the electronic structure is also studied by applying an electric field, and mechanical uniaxial and biaxial strain. The results show a significant change of the bandgaps and that an indirect-direct band-gap transition can be induced. This study highlights the positive prospect for the application of BiTeCl and BiTeBr layered sheets in novel electronic and energy conversion systems.

Nonvolatile tuning of the Rashba effect in the CuInP2S6/MoSSe/CuInP2S6 heterostructure

The van der Waals sandwich heterostructure CuInP2S6/MoSSe/CuInP2S6 (CIPS/MoSSe/CIPS) has first been employed as a prototype to tune the Rashba effect. By nonvolatile controlling of the orientation of the polarization of the top ferroelectric CIPS monolayer, it is confirmed that the Rashba effect can be switched on or off at the top position of the valence band (VB) around the Γ point. More significantly, we find that the Rashba coefficient increases by almost one order of magnitude for the “on” state as compared with the freestanding MoSSe monolayer. Based on the results of first-principle calculations, it is obtained that the enhancement of the Rashba effect results from the charge transfer from the top CIPS layer to the MoSSe layer or the bottom CIPS layer, and the lifting of the d-orbit band of the light Cu atom leads to the disappearance of Rashba spin splitting at the top of the VB around the Γ point. Furthermore, the polarization orientation of the bottom CIPS layer can greatly alter the bandgap of the sandwich structure. We hope the above nonvolatile and large amplitude tuning of the Rashba effect should be useful in the design of spintronic nano-devices.

Dipole control of Rashba spin splitting in a type-II Sb/InSe van der Waals heterostructure

InSe monolayer, belonging to group III–VI chalcogenide family, has shown promising performance in the realm of spintronic. Nevertheless, the out-of-plane mirror symmetry in InSe monolayer constrains the electrons' degrees of freedom, and this will confine its spin-related applications. Herein, we construct Sb/InSe van der Waals heterostructure to extend the electronic and spintronic properties of InSe. The density functional theory is utilized to verify the tunable electronic properties and Rashba spin splitting (RSS) of Sb/InSe heterostructure. According to the obtained results, the Sb/InSe heterostructure can be considered as a direct band gap semiconductor with typical type-II band alignment, where the electrons and holes are localized in the InSe and Sb layers, respectively. The RSS is recognized at conduction band minimum around Γ point in Sb/InSe, which is induced by the spontaneous internal electric field with electric dipole moment of 0.016 e Å from Sb to InSe. The vertical strain, in-plane strain, and external electric field are employed to modulate the strength of RSS. The Rashba coefficient and dipole moment exhibit the similar variation tendency, suggesting the strength of RSS depends on the magnitude of dipole moment. The controllable RSS makes Sb/InSe heterostructure become an appropriate candidate material for spintronic devices.

Strain induced Rashba splitting in CH3NH3PbBr3 organic–inorganic lead halide perovskite

Recently, it has been demonstrated that CH3NH3PbBr3 exhibits extraordinary giant Rashba splitting, which represents great opportunity for applications in spintronics and orbitronics. In this work, the strain induced Rashba band splitting of CH3NH3PbBr3 with various lattice constants from 5.4 Å to 6.4 Å has been investigated in detail by means of density functional theory including the spin–orbit coupling using full-relativistic pseduopotential. The calculation results indicate that the triple states at the conduction band maximum can be split by both spin–orbit coupling and strain. The details of the band structure at the conduction band maximum further reveal that the Rashba band splitting is strongly anisotropic along different directions due to the distortion of the Pb–Br octahedron. The maximum Rashba coefficient can be up to 2.0 eV Å at the equilibrium lattice constant. Furthermore, the Rashba coefficient is approximately linearly dependent on the strain in the range of ±5%. The Rashba coefficient can be even increased by nearly two times if CH3NH3PbBr3 is compressed by about 5%. Such strain induced Rashba coefficient enhancement may be useful for the application of CH3NH3PbBr3 in spintronic and orbitronic devices.

Nonlinear Control of Damping Constant by Electric Field in Ultrathin Ferromagnetic Films

The performances of many spintronic devices are governed by the damping constant and magnetic anisotropies of constituent materials. Spin-orbit coupling (SOC) plays a key role and is at the origins of these material parameters. Electric field control of magnetic anisotropy is in high demand for developing energy-efficient nanoscale spintronics devices. Although electric field control of interfacial magnetic anisotropies is well studied and understood, the damping constant, on the other hand, is conventionally controlled by current-induced spin-orbit torque. Here, we use an alternative approach to demonstrate nonlinear control of the damping constant in ultrathin ferromagnetic films by an electric field. We explicitly show that the presence of the Rashba SOC at a ferromagnet-insulator interface and the electric field dependence of the Rashba coefficient may account for the observed nonlinear behavior. Furthermore, we show that engineering of the underlying and oxide material properties, i.e., bulk SOC and Rashba SOC, to tune the spin angular momentum relaxation pathways, can possibly increase the device functionality significantly.

The Rashba Scale: Emergence of Band Anti-crossing as a Design Principle for Materials with Large Rashba Coefficient

The spin-orbit -induced spin splitting of energy bands in low symmetry compounds (the Rashba Effect) has a long-standing relevance to spintronic applications and to the fundamental understanding of symmetry breaking in solids, yet the knowledge of what controls its magnitude in different materials is difficult to anticipate. Indeed, rare discoveries of compounds with large Rashba coefficients are invariably greeted as pleasant surprises. We advance the understanding of the "Rashba Scale" using the "inverse design" approach by formulating theoretically the relevant design principle and then identifying compounds that satisfy it. We show that the presence of energy band anti-crossing provides a causal design principle of compounds with large Rashba coefficients, leading to the identification via first-principles calculations of 34 rationally designed strong-Rashba compounds. Since topological insulators must have band anti crossing, this leads us to establish an interesting cross functionality of "Topological Rashba Insulators" (TRI) that may provide a platform for the simultaneous control of spin splitting and spin-polarization.

Enhanced longevity of the spin helix in low-symmetry quantum wells

In a semiconductor, collective excitations of spin textures usually decay rather fast due to D'yakonov-Perel' spin relaxation. The latter arises from spin-orbit coupling, which induces wave-vector-dependent spin rotations that, in conjunction with random disorder scattering, generate spin decoherence. However, symmetries occurring under certain conditions can prevent the relaxation of particular homogeneous and inhomogeneous spin textures. The inhomogeneous spin texture, termed as persistent spin helix, is especially appealing as it enables us to manipulate the spin orientation while retaining a long spin lifetime. Recently, it was predicted that such symmetries can be realized in zinc-blende two-dimensional electron gases if at least two growth-direction Miller indices agree in modulus and the coefficients of the Rashba and linear Dresselhaus spin-orbit couplings are suitably matched [PRL 117, 236801 (2016)]. In the present paper, we systematically analyze the impact of the symmetry-breaking cubic Dresselhaus spin-orbit coupling, which generically coexists in these systems, on the stability of the emerging spin helices with respect to the growth direction. We find that, as an interplay between orientation and strength of the effective magnetic field induced by the cubic Dresselhaus terms, the spin relaxation is weakest for a low-symmetry growth direction that can be well approximated by a [225] lattice vector. These quantum wells yield a 30\% spin-helix lifetime enhancement compared to [001]-oriented electron gases and, remarkably, require a negligible Rashba coefficient. The rotation axis of the corresponding spin helix is only slightly tilted out of the quantum-well plane. This makes the experimental study of the spin-helix dynamics readily accessible for conventional optical spin orientation measurements where spins are excited and detected along the quantum-well growth direction.

Strain engineering of spin and Rashba splitting in Group-III monochalcogenide MX (M = Ga, In and X = S, Se, Te) monolayer

In this paper, spin properties of monolayer MX (M = Ga, In and X = S, Se, Te) in the presence of strain are studied. Density functional theory is used to investigate spin properties. The effect of spin-orbit coupling on the band gap is investigated, the results indicate the spin-orbit coupling has a higher effect in the compressive regime. Also, spin splitting in the conduction and valence bands respect to strain are compared for six materials. The location of conduction band minimum (CBM) imposed different type of spin splitting. These materials with mirror symmetry can display the Rashba effect while M valley is located at CBM. Strain tunes the conduction band minimum in three valleys (K, M and Γ valleys) and determines which spin effect (spin splitting, Rashba splitting or no spin splitting) has occurred in each strain for every material. Lastly, the relation between the Rashba parameter and the atomic mass is explored and it is observed that there is a linear correlation between atomic mass and Rashba coefficient.